// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_ATOMIC_SEQUENCE_NUM_H_
#define BASE_ATOMIC_SEQUENCE_NUM_H_

#include "base/atomicops.h"
#include "base/macros.h"

namespace base {

class AtomicSequenceNumber;

// Static (POD) AtomicSequenceNumber that MUST be used in global scope (or
// non-function scope) ONLY. This implementation does not generate any static
// initializer.  Note that it does not implement any constructor which means
// that its fields are not initialized except when it is stored in the global
// data section (.data in ELF). If you want to allocate an atomic sequence
// number on the stack (or heap), please use the AtomicSequenceNumber class
// declared below.
class StaticAtomicSequenceNumber {
public:
    inline int GetNext()
    {
        return static_cast<int>(
            base::subtle::NoBarrier_AtomicIncrement(&seq_, 1) - 1);
    }

private:
    friend class AtomicSequenceNumber;

    inline void Reset()
    {
        base::subtle::Release_Store(&seq_, 0);
    }

    base::subtle::Atomic32 seq_;
};

// AtomicSequenceNumber that can be stored and used safely (i.e. its fields are
// always initialized as opposed to StaticAtomicSequenceNumber declared above).
// Please use StaticAtomicSequenceNumber if you want to declare an atomic
// sequence number in the global scope.
class AtomicSequenceNumber {
public:
    AtomicSequenceNumber()
    {
        seq_.Reset();
    }

    inline int GetNext()
    {
        return seq_.GetNext();
    }

private:
    StaticAtomicSequenceNumber seq_;
    DISALLOW_COPY_AND_ASSIGN(AtomicSequenceNumber);
};

} // namespace base

#endif // BASE_ATOMIC_SEQUENCE_NUM_H_
